1. Technical Field
This invention relates to the use of catalyst compositions and systems for polymerization of olefin polymers.
2. Background of the Art
Knowledge of the molecular weight distribution (MWD) of a polymer provides information that is useful in predicting how the polymer will behave during compounding and forming processes. For some applications, such as certain films and fibers, it is desirable to employ a polymer, particularly an olefinic polymer that exhibits a relatively broad MWD.
It is known that some traditional Ziegler-Natta catalysts, which are multi-site or heterogeneous catalysts, can be used to produce olefinic polymers having a relatively broad MWD. It is also known that, in contrast, some of the recently commercialized single-site catalysts, such as metallocene catalysts, tend to produce polymer chains of similar length that exhibit a narrower MWD. Nonetheless, for some types of polymers metallocene catalysts offer particularly desirable properties including, for example, improved processing characteristics, reduced levels of solvent-soluble fractions, and improved strength and toughness not generally attainable with Ziegler-Natta catalysts.
Applicable industries have used various means to broaden the MWD of olefinic polymers, and particularly those catalyzed using single-site catalysts such as metallocenes. For example, multiple olefin polymerization reactors have been operated in series in order to expose the polymerization catalyst to multiple hydrogen levels. The result is a metallocene-catalyzed olefinic polymer with a broader MWD, but the costs of such multiple reactors and potentially the costs of corresponding plant redesign to employ such multiple reactors is often prohibitive.